Automatic lathes for metal-cutting machining of bar-shaped semi-finished products, which are referred to below as bar material, are basically known and can be basically divided into three different designs.
A so-called automatic short-turning lathe comprises a fixed spindle head in which a spindle with an associated drive unit is supported. On the side of the spindle head facing towards the process chamber of the automatic lathe a clamping means is provided, which is preferably embodied as a collet chuck or optionally as a jaw chuck. The spindle, which is horizontally supported and rotatably driven in the spindle head, is embodied as a hollow shaft so that the loading with the bar material of an automatic lathe of this type can be carried out via the rear side of the spindle head thereof. For loading, a so-called bar loader is provided on the rear side of the spindle head, which bar loader preferably is embodied as a bar loading magazine. With a bar loading magazine of this type, which is referred to below as a loading magazine, a material bar with its end to be machined is pushed from the rear side of the spindle head through the hollow spindle of the automatic lathe. The bar end is thereby clamped in the collet chuck such that the bar projects into the process chamber of the automatic lathe with an overhang, wherein the overhang essentially corresponds to a desired workpiece length. In the case of automatic short turning lathes, all of the adjusting, infeeding and forward feeding movements necessary for machining are carried out essentially by the carriage, which thus can be moved fully in all spatial directions, i.e., in the X, Y and Z direction.
An alternative design for an automatic lathe for processing bar material is represented by the so-called automatic long-turning lathe, which essentially differs from the automatic short-turning lathe previously described in two points.
For one thing, automatic long-turning lathes have a spindle head that is displaceable in the longitudinal direction, i.e., in the direction of the Z-axis of the automatic lathe. This displacement movement is performed by the drive axle of the spindle head, which drive axle can be controlled by the control of the automatic lathe, wherein as a rule so-called NC axes are used. The forward feed movement in the direction of the Z axis is thus carried out in the long-turning process by the spindle head movement against a tool fixed in the Z direction. The carriage of an automatic long-turning lathe thus performs only adjusting and infeed movements in the X and Y direction.
Furthermore, a category-defining automatic long-turning lathe comprises an additional fixed guide bushing, which is arranged between the spindle head that can be moved in the Z direction and the tool in the process chamber of the automatic lathe and ensures a stable guidance of the bar material to be processed. Since the bar material is held in the loading magazine by a clamping sleeve at the end of a push rod, a synchronization device is necessary in the loading magazine with which the loading magazine can be adapted to the movement of the entire bar material during the forward feed movement provided by the spindle head movement.
As a further design of an automatic lathe, according to recent developments a mixed form of the previously described automatic short-turning lathes and automatic long-turning lathes has become established on the market, which is referred to below as an automatic hybrid lathe. An automatic hybrid lathe of this type operates with a spindle head that is moveable in the longitudinal direction, in contrast to the automatic long-turning lathe, however, does not have a fixed guide bushing in the region of the tool so that the forward feed movement in the long turning process can be shown via the carriage as well as via the spindle head. Compared to the conventional automatic long-turning lathes, the spindle head path of which is limited to approx. 300 mm, automatic hybrid lathes therefore have a larger possible travel for the forward feed movement, which can be up to 450 mm.
The loading magazine respectively provided on an automatic lathe has to securely guide the bar material during the entire processing operation, wherein in this context the suppression of vibrations in the bar material has an essential function. The bar material to be processed is up to 6 m long and has a diameter between 1 mm up to 100 mm with round, tubular, hexagonal or square cross sections. Depending on the cross section and diameter, speeds of up to 15,000 1/min occur, which is why a vibration prophylaxis as well as an active suppression of vibrations is indispensible. These requirements are essentially met by a guidance of the bar material that is as exact as possible.
A particularly advantageous solution for such a guidance of the bar material that is as exact as possible is to support them in a rotatable manner in an oil-filled channel inside the loading magazine. With the rapid rotation of the bar material in the channel, the effect of a hydrodynamic bearing action occurs, whereby the bar material is guided in the loading magazine exactly and thus with low vibrations.
A weak point, however, is the transition region between the loading magazine and the automatic lathe. As a last support point for the bar material a lathe steady is therefore usually provided on the bar exit side of a loading magazine, which lathe steady comprises roller supports or bracket suppers that can be adjusted to the diameter of the bar material. Between the lathe steady of the loading magazine and the clamping means of the automatic lathe, however, there is a considerable transition region, over which the bar material remains without support and in which thus the development zones for vibrations form on the bar material. In the transition region between the loading magazine and the spindle head a transition tube is provided, which is embodied as a telescopic tube in the case of automatic long-turning lathes or automatic hybrid lathes.
In the case of a distance of approx. 400 mm between the loading magazine and the spindle head and a spindle length of approx. 800 mm, a transition region results in the order of magnitude of over 1,000 mm length, wherein with the automatic long-turning lathes and the automatic hybrid lathes the transition region can be increased by the path of the spindle head movement to up to 1,600 mm. The oscillatory behavior in the transition region depends essentially on the length of this transition region.
It is basically possible to improve the guide quality in that the diameters of the spindle and of the transition tube are reduced for thinner bar material, but economic reasons often argue against this, since very complex retooling work is necessary for changing workpiece diameters. For a reduction to smaller diameters various components which are in part difficult to access would have to be replaced in the automatic lathe and the loading magazine, if a drastic reduction in speed is not to be accepted. The latter leads to a marked loss of productivity as a rule.